Conclusion

We have found that Frankia infection leads to a dramatic increase of flavans biosynthesis in nodules compared to uninfected roots. We have shown the existence of regular pattern of cell layers containing flavans. Besides the flavan accumulation below the peridem, in the endoderm and between the nodule root and the apex, we have shown accumulation in cell layers of the cortex limiting compartment of infected and uninfected cells. The biological significance of this compartmentation might be to prevent the spread of the endophyte in the vascular bundle and in the nodule root. Alternatively, the flavan layers might contribute to the protection of secondary infection and/or the limitation of oxygen penetration in the nodule.